The composition and origin of the Iapetus dark material

In order to investigate the composition of the Iapetus dark material and the reasons for its peculiar distribution, telescopic observations, laboratory simulations, and photogeological studies are conducted. New telescopic spectra of superior accuracy and wavelength coverage for leading and trailing hemispheres are obtained over the range 0.3-2.6 microns. A mixing model is used to correct the leading-hemisphere spectra for the presence of polarcaps. It is shown that the spectral curve of pure Iapetus dark material is highly reddened in the visible, but gradually flattens near 2.0 microns. Structures along the bright-dark boundary, visible in Voyager 2 images, are inconsistent with an eruptive origin for the dark material; this zone is probably the region of grazing impacts of dust spiraling from Phoebe and the dark material is probably a minor component of the ice-dominated surface concentrated in the upper regolith produced by Phoebe-dust bombardment. An Iapetus-like bombardment regime may account for the hemispheric dichotomy of regolith properties on Callisto.